1. Field of the Invention
The present invention generally relates to the field of nonlinear optical phase conjugation, and more specifically to a phase conjugate mirror utilizing a novel and unique combination of Stimulated Brillouin Scattering (SBS) in an amplification regime and Four Wave Mixing (FWM) to realize an efficiency of at least 50% in producing a high energy, phase conjugate reflection with low spectral bandwidth.
2. Description of the Related Art
Nonlinear Optical Phase Conjugation (NOPC) involves the real-time spatial and/or temporal information processing of electromagnetic fields using Nonlinear Optical (NLO) techniques. Although the field has evolved to encompass a remarkably rich and diverse set of applications including nonlinear laser spectroscopy, the major thrust of NOPC lies in the area of real-time compensation of distortions encountered in electromagnetic fields due to propagation and/or transmission through various aberrating media.
A Phase Conjugate Mirror (PCM) produces a retro-reflection of an incident beam. The unique properties of phase conjugate reflection are illustrated in FIGS. 1a and 1b. FIG. 1a shows the case of a conventional plane mirror 10 illuminated by a monochromatic point light source 12. The mirror 10 merely changes the propagation direction of a diverging incident beam 14 shown in solid line to that of a reflected beam 16 shown in broken line in such a manner that the angles (not designated) of incidence and reflection are equal.
FIG. 1b illustrates the case of a phase conjugate mirror 18 which reflects the incident beam 14 in such a manner that a reflected beam 16' exactly retraces the incident beam 14 in a "time-reversed" sense. Wavefronts 22' of the reflected beam 16', shown in broken line, overlap wavefronts 20 of the incident beam 14, although shown as displaced in the drawing for illustrative purposes.
It will be noticed in FIG. 1a that the wavefronts 20 of the incident beam 14 appear concave as viewed from the source 12, whereas wavefronts 22 of the reflected beam 16 appear convex as viewed from the source 12. In FIG. 1b, both wavefronts 20 and 22' appear concave as viewed from the source 12. Due to this reversal of the phase of the wavefronts, the phenomenon is referred to as wave-front reversal (WFR) in Soviet literature.
The phase reversal effect is further illustrated in FIGS. 2a and 2b. In FIG. 2a, a planar wavefront 24 of a coherent light beam is passed through a distortion element such as a glass cylinder 26 which introduces a "bulge" 28a in a wavefront 28 of the beam during propagation through the cylinder 26. Reflection of the wavefront 28 from the plane mirror 10 produces a wavefront 30 having a bulge 30a which lags the remainder of the wavefront 30 in phase. Propagation of the wavefront 30 back through the cylinder 26 produces a wavefront 32 having a bulge 32a which is double the size of the bulges 28a and 30a. Where the cylinder 26 is replaced by a transmission medium such as the atmosphere in an aerospace application or by an amplifier, the effect is especially undesirable. Any distortion introduced by an intervening medium or amplifier is doubled during two passes therethrough.
The phase conjugate mirror 18 illustrated in FIG. 2b has the highly desirable property of reversing and thereby eliminating any distortion introduced by a medium represented by the cylinder 26. Propagation of the wavefront 24 through the cylinder 26 produces the distorted wavefront 28 as in FIG. 2a. However, the phase of a wavefront 34 reflected by the phase conjugate mirror 18 is reversed with reference to the incident wavefront 28 in that a bulge 34a leads the remainder of the wavefront 34 in phase in a manner opposite to that of the wavefront 30 shown in FIG. 2a. Propagation of the wavefront 34 through the cylinder 26 produces a wavefront 36 which is planar (equiphase), since a lagging bulge (not shown) introduced by the cylinder 26 cancels the leading bulge 34a of the wavefront 34. It is this property of phase conjugate reflection which is so desirable since it enables propagation through and amplification by intervening media without distortion thereby.
Two widely used methods for producing optical phase conjugation are Stimulated Brillouin Scattering and Four Wave Mixing (FWM). Phase conjugation by stimulated Brillouin scattering was first recognized by researchers at the Lebedev Physical Institute in Moscow in 1972, as documented in Zel'dovich, B. Ya et al, Pis'ma Zh. Eksp. Teor. Fiz. Pisma 15, 160 [JETP Lett. (Engl. Transl.) 15, 109](1972). A general introduction to the principles of optical phase conjugation is found in "Optical Phase Conjugation", by Vladimir V. Shkunov and Boris Ya. Zel'dovich, Scientific American, Dec. 1985, pp. 54-59. An introduction to practical application of optical phase conjugation including detailed presentations of stimulated Brillouin scattering and four wave mixing is found in "Applications of Optical Phase Conjugation", by David M. Pepper (one of the present inventors), Scientific American, Jan. 1986, pp. 74-83. A detailed treatise on the optical, physical and mathematical principles of optical phase conjugation is found in "Non-Linear Optical Phase Conjugation", by David M. Pepper, Laser Handbook Vol. 4, Elsevier Science Publishers BV, North-Holland Physics Publishing, Amsterdam, 1985. A basic reference to Brillouin-enhanced four-wave mixing, the specific type of four wave mixing relevant to this invention, is found in a paper entitled "Wave-front inversion of weak optical signals with a large reflection coefficient", by N. F. Andreev et al, JETP Lett. Vol. 32, No. 11, 5 Dec. 1980, pp. 625-629. This reference also teaches how all of the interacting beams required in the four wave mixing process may be derived from a single coherent laser source by means of beam splitting.
Laser radar is of increasing importance in a number of modern military applications such as missile guidance, terrain following and obstruction avoidance, global atmospheric measurements and as elements in Strategic Defense Initiative (SDI) systems. Present laser radar concepts exclusively employ CO.sub.2 lasers, which are limited in lifetime (particularly in space-based scenarios).
There is a need for a narrow bandwidth, high power laser source for laser radar applications. Various limitations have heretofore precluded the use of phase conjugate mirrors in applications including laser radar which require narrow source bandwidths of approximately 10 KHz to 1 MHz. The spectral bandwidth of a conjugate beam produced by a basic (SBS) phase conjugate mirror can be no less than approximately one-tenth of the Brillouin gain linewidth. This minimum spectral bandwidth limit results from the phase jumps introduced by the random nature of the acoustic noise that creates and sustains the Brillouin process. Typical Brillouin linewidths range from approximately 5 MHz to 300 MHz for a pump laser wavelength of one micrometer. Four wave mixing eliminates the phase jumps and enables narrow linewidths, but requires auxiliary coherent light sources to produce the reference means which must be more powerful than the beam being conjugated and of nearly diffraction-limited optical quality, thereby greatly increasing the cost and complexity of the apparatus. In principle, one can realize a high-energy, narrow modulation linewidth optical system in several ways. First, the output of a high-energy laser can be directly modulated with a narrow linewidth modulator. This scheme is undesirable since the modulator must be placed in the high-energy output leg of the system and must therefore be resilient in terms of laser damage, stress-induced birefringence, and other mechanical and thermal perturbations. All of these problems can degrade the performance of the system by introducing phase aberrations and imposing various deleterious nonlinear optical distortions onto the output beam. Another approach is to modulate a narrowband, low-energy oscillator, which then drives a laser amplifier chain. This scheme also has drawbacks: even though the modulator is in a low-energy leg of the system, the final high-energy output can be distorted by the amplifier(s) themselves. Distortions such as stress-induced birefringence, thermal, mechanical, beam wander, etc. can be imposed onto a laser beam as it traverses a series of amplifiers.